Active noise cancellation using frequency response control

Abstract

An active noise cancellation system ( 20 ) includes using a frequency domain response of a secondary path ( 44 ) of the system. The frequency domain response information provides an indication of a desired magnitude and phase of a reference signal that is used for generating a cancellation signal that drives a speaker ( 28 ) to achieve the desired amount of noise cancellation. Using a frequency domain response greatly reduces the number of computations required and the amount of memory used within the system controller. In one example, a reference signal adjustment module ( 40 ) applies the desired magnitude and phase adjustments to a reference signal having a known frequency. In another example, the referenced signal tone generator ( 22, 40 ) incorporates the desired amplitude and phase in the reference signal.

I claim:  
     
         1 . A method of generating a reference signal in a noise cancellation system having a path between a generator of a noise cancellation signal and a sound detector that detects the effect of the noise cancellation signal, comprising the steps of: 
 determining a frequency domain response of the path for at least one signal of a known frequency; and    adjusting the phase and amplitude of the reference signal responsive to the determined frequency domain response.    
     
     
         2 . The method of  claim 1 , including generating a plurality of reference signal tones each having a known frequency and adjusting the amplitude and phase of each tone, individually.  
     
     
         3 . The method of  claim 1 , including using the adjusted reference signal to generate a noise cancellation signal.  
     
     
         4 . The method of  claim 1 , including determining an amplitude adjustment value and applying the determined value to the reference signal amplitude and determining a phase shift amount and shifting the phase of the reference signal by the determined amount.  
     
     
         5 . The method of  claim 1 , including applying a gain factor to each of a plurality of components of the reference signal comprising a sine component and a cosine component to achieve the adjusted phase and amplitude of the reference signal.  
     
     
         6 . The method of  claim 1 , wherein the system includes a tone generator that generates the reference signal and wherein the adjusting occurs in the tone generator.  
     
     
         7 . The method of  claim 1 , wherein the system includes a tone generator that generates the reference signal with a consistent phase and amplitude and wherein the adjusting occurs subsequent to the tone generator generating the reference signal.  
     
     
         8 . The method of  claim 1 , wherein the system includes a tachometer that provides information regarding an RPM value of a vehicle engine and wherein the reference signal frequency corresponds to the RPM value.  
     
     
         9 . The method of  claim 1 , including determining a time domain response of the path and determining the frequency domain response by applying a Fast Fourier Transform to the time domain response.  
     
     
         10 . The method of  claim 1 , including operating in a first mode for determining the frequency domain response and operating in a second mode while adjusting the phase and amplitude of the reference signal and including a delay between the first and second modes.  
     
     
         11 . An active noise cancellation system, comprising: 
 a speaker;    a microphone that detects a combination of a sound from the speaker and noise in the system;    a controller having a first module that provides a frequency domain response of the portion of the system associated with the speaker and microphone, a tone generator that generates at least one reference signal having a selected frequency and a second module that adjusts the phase and amplitude of the reference signal responsive to the frequency domain response information.    
     
     
         12 . The system of  claim 11 , wherein the tone generator generates a reference signal having unity amplitude and zero phase and wherein the second module adjusts the amplitude and phase of the reference signal subsequent to the reference signal being generated by the tone generator.  
     
     
         13 . The system of  claim 11 , wherein the tone generator includes the second module such that the reference signal provided by the tone generator includes the desired amplitude and phase.  
     
     
         14 . The system of  claim 11 , including a cancellation signal generating module that utilizes the reference signal to provide a cancellation signal for driving the speaker.  
     
     
         15 . The system of  claim 11 , including a Finite impulse response filter that provides a time domain response of the portion of the system associated with the speaker and the microphone and wherein the first module applies a Fast Fourier Transform to the time domain response to provide the frequency domain response.  
     
     
         16 . The system of  claim 11 , wherein the controller operates in a first mode for determining the frequency domain response and operates in a second mode at a different time for applying the phase and amplitude adjustments to the reference signal.  
     
     
         17 . The system of  claim 15 , including a delay between the controller operating in the first mode and the second mode and, wherein the controller converts between a determined time domain response of the portion of the system associated with the speaker and the microphone and the frequency domain response during the delay.